In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities or dopants. Ion implanters are used to treat silicon wafers with an ion beam, in order to produce n or p type extrinsic material doping or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductors, the ion implanter injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in “n type” extrinsic material wafers, whereas if “p type” extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted.
Typical ion implanters include an ion source for generating positively charged ions from source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway can be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with gas molecules.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by a magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the wafer or beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway which will effectively separate ions of different charge-to-mass ratios.
For some ion implantation systems, the physical size of the beam is smaller than a target workpiece, so the beam is scanned in one or more directions in order to adequately cover a surface of the target workpiece. Generally, an electric or magnetic based scanner scans the ion beam in a fast scan direction and a mechanical device moves the target workpiece in a slow scan direction.
An electric scanner, in one example, comprises two electrodes to which time varying voltages are applied. The electric scanner creates a time varying electric field that diverts or alters the path of the ion beam, such that the ion beam after the scanner appears to originate from a vertex point. A parallelizer then redirects the ion beam along a path parallel to its original path.
One drawback to an electric scanner is that it can cause space-charge blowup and thereby limit the amount of beam current that can be delivered to a target workpiece. Beam blow-up is the increase of transverse velocity of a beam with distance along the beam path or axis, and space-charge blowup is beam blow up caused by beam-internal space-charge forces, which are proportional to a beam perveance. During ion beam drift in electric field free regions the ion beam attracts electrons generated from ionizing collisions with background gas or from secondary electron emission from collisions with aperture, for example, a process referred to as self-neutralization. Nonzero fields of the electric scanner remove the electrons from self-neutralization and cause the beam within the scanner to blow up, which can result in a beam envelope unmanageably large and thus beam current loss.
A magnetic scanner generates a time varying magnetic field through which the ion beam passes. The time varying magnetic field diverts or alters the path of the ion beam such that after the scanner the ion beam appears to originate from a vertex point. A parallelizer is then employed to bend the beam to a direction parallel to the ion beam prior to scanning.
The magnetic scanner does not suffer from the space-charge blow-up resulting from electric fields like the electric scanner. As a result, using a magnetic scanner instead of an electric scanner can permit higher beam currents to be obtained. However, it has been noted that the ion beam goes through an anomalous transport phase when the magnetic field in the scanning magnet has zero or close to zero amplitude. This effect is referred to as a zero-field effect (ZFE), caused by a multitude of simultaneous effects including cyclotron/betatron effects on self-neutralizing electrons as well as enhanced ion beam neutralization by free electron motion.
One technique to account for the anomalous transport phase is to immerse the magnetic scanner into a secondary magnetic field to prevent electron motion, such as described in Glavish et al., U.S. Pat. No. 5,481,116, filed Jun. 10, 1994. The presence of the second magnetic field prevents the anomalous transport phase by always having a non-zero magnetic field applied to the ion beam. However, this technique adds complexity to the scanner assembly in terms of adding a secondary magnetic circuit. Additionally, it immerses the complete scanner volume in a minimum field whose effects on beam transport, i.e. steering and focusing of an additional magnetic field, have to be taken into account, which increases the complexity of the beamline design. Furthermore, the presence of the second magnetic field suppressed self-neutralization and can reduce or limit beam current transmitted through the scanner.